Distillation
... A Separation Method
Background Concepts - Definitions
• Vapor Pressure –
Gas pressure created by the molecules
of a liquid, which have acquired sufficient
Kinetic Energy to escape to the vapor
phase.
• As Temperature increases, the average
Kinetic Energy and the Vapor Pressure
increase until the boiling point is reached.
Background Concepts (definitions)
• Boiling Point – The temperature at which
the vapor pressure of a liquid equals the
pressure applied by the surroundings to
the liquid.
• Boiling points for ethanol ...
Pressure = 760 torr B Pt = 78 oC
Pressure = 600 torr B Pt = 73 oC
Pressure = 10 torr B Pt = -2 oC
Background Concepts (Relationships)
• For two pure liquids, which have
different boiling points…
.... the vapor pressure at a given
temperature will be higher for the liquid
of lower boiling point.
Background Concepts
• The temperature of the vapors above a
boiling liquid will remain constant at the
boiling point.
• At the boiling point for a pure liquid, the
liquid and vapor (gas) phases are in
equilibrium.
• The boiling point (at a specified pressure)
is a characteristic property of a pure
liquid.
Background – Raoult’s Law
• For an ideal solution with a volatile
solvent and a non-volatile solute
(e.g. water and salt) …
VPsolution = Xsolvent x VPsolvent
As you increase the concentration of the non-volatile
solute, you decrease the vapor pressure of the solution
and increase the boiling point.
… hence “Boiling Point Elevation”
So What ??
For an ideal solution with a volatile
solvent and a non-volatile solute
(e.g. water and salt) …
…separation of the two components
is as simple as heating the mixture and
collecting and condensing the vapor.
Background – Dalton’s Law
• For an ideal solution made of two volatile
solvents, the total vapor pressure is the
sum of the vapor pressures of the two
solvents.
• For a solution of liquid ‘a’ and liquid ‘b’
VPtotal = Xa (VPa) + Xb (VPb)
Deviations from Dalton’s Law
• For non-ideal solutions, the combined
vapor pressure may be either higher
(positive deviation) or lower (negative
deviation) than predicted by Dalton’s Law.
• When VP is higher, means BP is lower…
• When VP is lower, means BP is higher…
…than predicted by Dalton’s Law.
Deviations from Dalton’s Law
• For non-ideal solutions, there is either strong
forces of attraction (negative deviation) or
repulsion (positive deviation) between the
molecules of the different components of the
solution.
– Attractive forces = components held together more strongly =
lower VP = requires more heat = higher BP
– Repulsive forces = components not held together = higher VP =
requires less heat = lower BP
• A non-ideal solution may distill with a constant
boiling point as if it were one pure substance =
Azeotrope.
Positive Deviation
• Classic example: ethanol and water
Pure ethanol – B. Pt of 78 oC
Pure water – B. Pt of 100 oC
Mixture of ethanol (95 %) and water (5 %)
distills with a constant B. Pt. of about 75 oC.
Background Concepts
• For an ideal solution of two miscible
liquids of different boiling points, the
composition of the liquid and vapor
phases is not the same.
• The vapor will contain more of the liquid
with the higher vapor pressure or lower
boiling point.
(X’s and O’s represent percentage of each component at each level.)
essentially pure "O"
O O O O temp 50
O
O
O O OO
Po = 0.9 X 833 mmHg = 750 mmHg
Px = 0.1 X 100 mmHg = 10mmHg
760 mmHg
Po = 0.8 X 855 mmHg = 684 mmHg
Px = 0.2 X 380 mmHg = 76 mmHg
760 mmHg
Po = 0.7 X 867 mmHg = 608 mmHg
Px = 0.3 X 507 mmHg = 152 mmHg
760 mmHg
Po = 0.6 X 887 mmHg = 532 mmHg
Px = 0.4 X 570 mmHg = 228 mmHg
760 mmHg
Po = 0.5 X 912 mmHg = 456 mmHg
Px = 0.5 X 608 mmHg = 304 mmHg
760 mmHg
O O O O temp 60
O
O
O O XO
750/760 X 100 = 99% O
10/760 X 100 = 1% X
O O O O temp 70
O
X
O O XO
684/760 X 100 = 90% O
76/760 X 100 = 10% X
O X
O temp 80
O
O
X O OX
O
O XO O
X
X temp 90
O
O X O
608/760 X 100 = 80% O
152/760 X 100 = 20% X
532/760 X 100 = 70% O
228/760 X 100 = 30% X
456/760 X 100 = 60% O
304/760 X 100 = 40% X
O X X O
O
O
X
O
O
X X
X X temp 100
X OO
Phase Diagram (Liquids A and B)
http://www.uwlas.edu/faculty/koster/Distillation305.htm
Terms and Definitions
• Theoretical Plate – one of the horizontal
lines in the previous graph or 1 simple
equilibration between the liquid and
vapor phase.
Simple Distillation
(Use a 10-mL graduated cylinder to collect fractions)
http://www.uwlas.edu/faculty/koster/Distillation305.htm
Simple Distillation
http://www.uwlas.edu/faculty/koster/Distillation305.htm
Fractional Distillation
• Increasing the surface
area that the vapors contact
between the liquid and the
condenser, increases the
number of theoretical
plates.
http://www.uwlas.edu/faculty/koster/Distillation305.htm
Fractional Distillation
• Greater plates = greater purity of distillate
= sharper transition in distillation plot
• The number of Theoretical Plates or the
height equivalent to a theoretical plate
(HETP) is a measure of the efficiency of
a column to separate components.
Preparing your NB…
• It would be helpful to read pgs. 53-57 (“Simple
Distillation, Miniscale Apparatus”) and pgs. 131132 (in addition to Sec. 4.1 and 4.2)
• Figure 2.37‒(a) on pg. 54!
• 2 chemicals to include in Table of Properties:
ethyl acetate & n-butyl acetate
• No chemical reactions, this is just separation
• Procedure – you can print the procedural steps
that are in the handout (on Moodle).
Results
After collecting the temperature data,
prepare a graph of “Temperature (y-axis) vs.
Volume (mL) of Distillate Collected (x-axis)”
and put it in the Calculations section of your
NB. (see slide #17 for a sample of what a
good simple distillation looks like)
Remember – a graph must be drawn using either
graph paper or a computer – It can not simply be
“guesstimated” on the notebook page.
Discussion
• Results = Graph of “Temperature vs. Volume
(mL) of Distillate collected”
– How similar does your graph look to the one that’s on
slide #17?
– Ideally, you’d want the temperature transition (from
low temp. to higher temp.) to be as sharp as possible in
order to minimize collection of a mixture of ethyl and
butyl acetates.
• Continue writing discussion as outlined in
the syllabus.